Behavioral feedback regulation of circadian rhythm phase angle in light-dark entrained mice

Induced and spontaneous wheel running can alter the phase and period (tau) of circadian rhythms in rodents. The relationship between spontaneous runni ng and the phase angle (psi) of entrainment to 24-h light-dark (LD) cycles was evaluated in C57BL/6j mice. With a wheel freely available, psi was sign ificantly correlated with the absolute (r = 0.32) and relative (r = 0.44) a mount of activity during the first 2 h of the activity period. When wheels were locked during the first half of the night in LD and then unlocked in c onstant dark (DD), mice exhibited a delayed psi and lengthened tau compared with mice that had wheels locked during the second half of the night. In D D, tau correlated negatively with total daily activity. To evaluate if whee l running modulates the phase-resetting actions of LD, phase shifts to ligh t pulses were measured at two time points in DD, when daily activity levels differed by 40%. Phase delays to light were 56% greater when activity leve ls were lower. However, in a counterbalanced follow-up experiment, phase ad vances and delays to light pulses were not affected by the availability of wheels, although an effect of time in DD was replicated. Spontaneous activi ty can regulate psi and tau without altering the response of the pacemaker to light.